The influence of the chemical nature of high-solid organic wastes (HSOW) on their biohydrogen generation was investigated using simulated high-solid bioreactors under mesophilic conditions. The bioreactors were filled with 10% total solid of rice, potato, fat meat, chicken skin, egg, and lean meat. Experimental results indicate that hydrogen-producing potential of carbohydrate-rich HSOW (rice and potato) was approximately 20 times larger than that of fat-rich HSOW (fat meat and chicken skin) and of protein-rich HSOW (egg and lean meat). According to development trends of pH and hydrogen, pH around 6.0 might be threshold for heat-shock digested sludge; that is Clostridium-rich sludge, converting fat- and protein-rich HSOW to hydrogen; but pH threshold for Clostridium-rich sludge consuming carbohydrates-rich HSOW occurred at around 5.0. In bulk solution, volatile fatty acids (VFA) and alcohols occurred concurrently and the trends of carbohydrate-rich HSOW were similar to those of protein-rich HSOW. Considering developments of carbohydrates and VFAs together with that of hydrogen one infers that lipids would be hydrolyzed to carbohydrates and the carbon flow would proceed through acetate/H-2 + CO2 cleavage. Indications from cluster analysis of pH development trends are that a cometabolism would be obtained in wastes rich in carbohydrate and protein. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.